1. Field of the Invention
The present invention relates to an exhaust purification device for an engine, and more specifically, to an exhaust purification device in which a reducing agent injected by reducing-agent supply means is supplied to an after-treatment device together with exhaust gas agitated by agitating means.
2. Description of the Related Art
Some exhaust purification devices for removing harmful components contained in exhaust gas by using a reducing agent of the above-mentioned type are provided, for example, with selective reduction-type NOx catalysts (hereinafter, referred to as SCR catalysts). An exhaust purification device with an SCR catalyst is disclosed, for example, in Unexamined Japanese Patent Application Publication No. 2006-29233 (hereinafter, referred to as Document 1).
In the exhaust purification device of Document 1, an oxidizing catalyst is interposed in the exhaust passage of the engine, and the SCR catalyst is set downstream of the oxidizing catalyst. In an exhaust pipe connecting the oxidizing catalyst and the SCR catalyst, there is disposed an injection nozzle that injects urea-water as a reducing agent. A fin device is also disposed in the exhaust pipe on the upstream side of the injection nozzle.
The fin device is fabricated by press-molding a disk-shaped steel base plate. This base plate is arranged in the exhaust pipe to divide the exhaust pipe into upstream and downstream sides. In the base plate, a plurality of fins, which are arranged around the axis of the base plate, are formed by being bent to be raised from the base plate at predetermined angles in a downstream direction. At the same time, through holes with shapes matching with their respective fins are accordingly formed in the base plate. The fin device guides the exhaust gas that has passed through the through holes along the inclinations of the fins, and thus creates a swirl flow in the exhaust gas. The urea-water injected from the injection nozzle is diffused into the exhaust gas by this swirl flow.
In the exhaust purification device thus configured, exhaust pressure is constantly applied onto the base plate of the fin device, so that the base plate needs to be firmly fixed within the exhaust pipe. For example, the outer circumference of the base plate is welded within the exhaust pipe at regular intervals. FIG. 13 is an enlarged partial view of the fin device of Document 1 as viewed from an exhaust upstream side. In this example, a fin device 101 is disposed in a position of a flange 13a formed in the exhaust pipe. The outer circumference of a base plate 22 is welded to the flange 13a at places toward which spoke portions 24 supporting fins 23 extend. Weld beads 32 are formed in these places.
The exhaust purification device is exposed to exhaust heat during engine operation and greatly rises in temperature compared with a cold state of the engine. Arrival temperature is varied, depending upon the members constructing the exhaust purification device. For example, in the case of the exhaust purification device installed in a vehicle, a casing that accommodates the oxidizing catalyst and the SCR catalyst is cooled by outside air, and is also exposed to the running wind, depending upon its installation location. For these reasons, a temperature rise in such an exhaust purification device is small. At the same time, the oxidizing catalyst and the SCR catalyst accommodated inside are not cooled by the outside air, and are therefore remarkably increased in temperature. Such a temperature gap is a favorable phenomenon when the catalysts should be kept at activation temperatures.
However, a similar tendency can be seen in the exhaust pipe and the fin device. In other words, the arrival temperature of the exhaust pipe cooled by the outside air is relatively low, whereas the arrival temperature of the fin device located inside is much higher than that of the exhaust pipe. Especially in the configuration illustrated in FIG. 13, the flange 13a having large cooling area is suppressed in temperature rise by the outside air. In result, a temperature gap between the flange 13a and the fin device 101 is enlarged.
In addition, if a DPF (diesel particulate filter) for collecting particulates contained in exhaust gas is set upstream of the fin device 101, during the forcible regeneration in which the particulates are burned by feeding the unburned fuel to the DPF, the fin device 101 located downstream of the DPF is increased in temperature by being applied with the heat of the DPF raised in temperature. In this case, the temperature gap is further enlarged.
For example, when the DPF is forcibly regenerated in a configuration where the fin device 101 is disposed at the flange 13a of the exhaust pipe, the flange 13a has a temperature in a range of from about 300 to about 400 degrees centigrade, whereas the temperature of the fin device 101 located inside rises up to about 600 degrees centigrade. Because of this temperature gap, the flange 13a and the fin device 101 are significantly different from each other in thermal expansion. Compared to the exhaust pipe and the flange 13a, which form the outer shell of the exhaust purification device, a steel plate forming the fin device 101 that changes an exhaust gas flow is thin and low in rigidity. The fin device 101 is thermally expanded in a state constrained in its outer circumference by the exhaust pipe and the flange 13a. This causes the problem that the stress produced during thermal expansion is concentrated on the center of the fin device 101 and the like, and then that the fin device 101 is damaged by so-called thermal stress.